The majority of crab species belongs to the infra-order Brachyura, which means "short tails". They have 5 pairs of walking legs. The first pair is modified to have large claws (chelae) used for food capture, feeding and fighting. The 5th pair may be slightly modified as paddle-like swimming legs in the family Portunidae. In the families Dromiidae and Dorippidae the 5th pair of legs are short and directed towards the dorsal surface to hold on to pieces of sponge, leaves, shells or other kinds of material, which are used to cover the body.
The infra-order Anomura ("abnormal tail") includes the familiar hermit crabs (superfamily Paguroidea), the commensalistic porcelain crabs (family Porcellanidae), the squat lobsters (family Galatheidae), and the large king crabs (family Lithodidae).
In the Brachyura as well as some Anomura the tail is small and bent ventrally to fit into a depression of the ventral (sternal) plates.
For terminology of crab morphology see: http://www.bluecrab.info/anatomy.html
Brachyuran crabs show several sexual dimorphies. In all species males have a narrow abdomen and the 1st pair of abdominal appendages is modified as gonopods for transferring sperm. The abdomen of adult females is broadly rounded, almost as wide as the ventral surface of the body. The pleopods (abdominal appendages) are used to hold the fertilized eggs during the brooding period. Immature females have an abdomen that is wider than that of the males, but narrower than that of sexually mature females. In many species the claws of the males are much bigger than those of the females. The males use the enlarged claws for display, defence or for fighting as well as for food handling. Often one claw is bigger than the other; this may be the case in both sexes. Size, shape, ornamentation and colour of chelae may be important for species identification.
The carapace in Brachyurans is divided by more or less distinct grooves into a number of sections. These may be important for species identification. Also the number, size and position of spines on the carapace are important. The shape of the male gonopod is one of the most important characters for species identification, and many older descriptions of new species are based on only male characters. Another useful character is the 3rd maxillipeds, which form little "doors" covering the other mouth appendages.
Crabs are among the most common marine invertebrates, and also among the most common introduced and invasive species. Several introduced species already occur in one or more of the Nordic countries, and several more introduced species may be spreading from neighbouring countries. The introduced crabs vary enormously in size as well as impact, from the small (< 2cm) American mud crab, Rhithropanopeus harrisii, to the more than 20 cm anomuran king crab, Paralithodes camtschaticus. The introduced species also show great differences in tolerance of temperature and salinity, and their reproductive potential and life history show differences related to their size and origin.